NETWORK ANALYSES OF THE DINOSAUR-DOMINATED ECOSYSTEM OF THE UPPER JURASSIC MORRISON FORMATION, WESTERN INTERIOR NORTH AMERICA

We compiled data from the literature on the biota of the Upper Jurassic Morrison Formation to perform network analyses of food web structure through six previously recognized biostratigraphic zones. Our goals were to compare results to network analyses of modern terrestrial ecosystems and to test the hypothesis that biostratigraphic zone boundaries correspond to temporal changes in food-web structure. Results show that food-web structure remained essentially unchanged through the period of deposition represented by the five stratigraphically highest biostratigraphic zones (biozones 2–6). The paleocommunities of these five biozones are extremely resistant to disruptions caused by extinctions of random taxa, as well as losses of taxa that are most highly connected by feeding links to other organisms in each food web. Food webs for biozones 2–6 are more vulnerable to effects of extinctions of the least connected taxa, a pattern that is known from only one modern terrestrial food web––that of Serengeti National Park in Tanzania, Africa; the Serengeti food web may be considered an analog for the Morrison Formation ecosystem in this respect. The robustness of the Morrison Formation ecosystem represented by the basal-most biozone (biozone 1) is opposite that of biozones 2–6, i.e., it is most susceptible to loss of the most connected taxa. This result, along with food-web properties that are substantially different from those of biozones 2–6, suggest that the initial Morrison Formation ecosystem underwent a significant change at the boundary between biozones 1 and 2. This agrees with previously published paleoecological analyses of Morrison Formation biota.